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Base Transceiver Station

Electronics & communication Eng. Institute of technology, Nirma University Page 1

IntroductionA base transceiver station or cell site (BTS) is a piece of equipment that facilitates

wireless communication between user equipment (UE) and a network. UEs are

devices like mobile phones (handsets), WLL phones, computers with wireless

internet connectivity, WiFi and WiMAX gadgets etc. The network can be that ofany of the wireless communication technologies like GSM, CDMA, WLL, WAN,

WiFi, WiMAX etc. BTS is also referred to as the radio base station (RBS), node B(in 3G Networks) or, simply, the base station (BS). For discussion of the LTE

standard the abbreviation eNB for enhanced node B is widely used.So, to

understand BTS we have to go through the basics of GSM system because BTS is

a part of a GSM system.

Various Subsystems of GSM

Figure 1 Subsystems of GSM

Base Station Subsystem (BSS)

Figure 2 Subsystems of BSS

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From this figure we can say that BTS is a part of BSS(Base station subsystem)

The Base Station Subsystem (BSS)

The Base Station Subsystem consists of the following elements:1. BSC (Base Station Controller)

2. BTS (Base Transceiver Station)3. TC (Transcoder)

Figure 3 Working of BSS

The Base Station Controller (BSC) is the central network element of the BSS anditcontrols the radio network. This means that the main responsibilities of the BSC

are: Connection establishment between MS and NSS, Mobility management,Statisticalraw data collection, Air and A interface signalling support.

The Base Transceiver Station (BTS) is a network element maintaining the Air

interface. It takes care of Air interface signalling, Air interface ciphering andspeechprocessing. In this context, speech processing refers to all the functions the

BTSperforms in order to guarantee an error-free connection between the MS andthe BTS.The TransCoder (TC) is a BSS element taking care of speech

transcoding, i.e. it iscapable of converting speech from one digital coding format to

another and vice versa.

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The BTS, BSC and TC together form the Base Station Subsystem (BSS) which is a

part of the GSM network taking care of the following major functions:

Radio Path Control

In the GSM network, the Base Station Subsystem (BSS) is the part of the networktaking care of Radio Resources, i.e. radio channel allocation and quality of the

radioconnection. For this purpose, the GSM Technical Specifications define about120different parameters for each BTS. These parameters define exactly what kind

of BTSis in question and how MSs may "see" the network when moving in thisBTS area.The BTS parameters handle the following major items: what kind of

handovers (whenand why), paging organization, radio power level control and

BTS identification.

BTS and TC Control

Inside the BSS, all the BTSs and TCs are connected to the BSC(s). TheBSCmaintains the BTSs. In other words, the BSC is capable of separating (barring)a BTSfrom the network and collecting alarm information. Transcoders are also

maintainedby the BSC, i.e. the BSC collects alarms related to the

Transcoders.SynchronizationThe BSS uses hierarchical synchronization whichmeans that the MSC synchronizesthe BSC and the BSC further synchronizes the

BTSs associated with that particularBSC. Inside the BSS, synchronization is

controlled by the BSC. Synchronization is acritical issue in the GSM network dueto the nature of the information transferred. Ifthe synchronization chain is not

working correctly, calls may be cut or the call qualitymay not be the best possible.

Ultimately, it may even be impossible to establish a call.

Air &A Interface Signalling:

In order to establish a call, the MS must have a connection through the BSS. This

connection requires several signalling protocols.

Connection Establishment between MS and NSS

The BSS is located between two interfaces, the Air and the A interface. From the

callestablishment point of view, the MS must have a connection through these

twointerfaces before a call can be established. Generally speaking, this connectionmay beeither a signallingtype of connection or a traffic (speech, data) type of

connection.

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Network Switching Subsystem (NSS)

The elements of Network Switching Subsystem that have been described so farare:

MSC (Mobile Services Switching Centre)

VLR (Visitor Location Register) HLR (Home Location Register)

The MSC is responsible for controlling calls in the mobile network. It identifies theorigin and destination of a call (either a mobile station or a fixed telephone in both

cases), as well as the type of a call. An MSC acting as a bridge between a mobilenetwork and a fixed network is called a Gateway MSC. An MSC is

normallyintegrated with a VLR, which maintains information related to the

subscribers who arecurrently in the service area of the MSC. The VLR carries outlocation registrationsand updates. The MSC associated with it initiates thepaging

process.

A VLR database is always temporary (in the sense that the data is held as long asthesubscriber is within its service area), whereas the HLR maintains a permanent

registerof the subscribers. In addition to the fixed data, the HLR also maintains atemporarydatabase which contains the current location of its customers. This data

is required forrouting calls.

In addition, there are two more elements in the NSS: the Authentication Centre

(AC)and the Equipment Identity Register (EIR). They are usually implemented as

part ofHLR and they deal with the security functions.

Network Management SubsystemThe Network Management Subsystem (NMS) is the third subsystem of the GSM

network in addition to the Network Switching Subsystem (NSS) and Base Station

Subsystem (BSS). The purpose of the NMS is to monitor various functions

andelements of the network. These tasks are carried out by the NMS/2000 whichconsistsof a number of Work Stations, Servers and a Router which connects to a

DataCommunications Network (DCN).The operator workstations are connected to the database and communication

serversvia a Local Area Network (LAN). The database server stores the

managementinformation about the network. The communications server takes careof the datacommunications between the NMS and the equipment in the GSMnetwork known as Network Elements These communications are carried over a

Data CommunicationsNetwork (DCN) which connects to the NMS via a router.The DCN is normallyimplemented using an X.25 Packet Switching Network.

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The functions of the NMS can be divided into three categories:

Fault Management

Configuration Management

Performance Management

These functions cover the whole of the GSM network elements from the level of

individual BTSs, up to MSCs and HLRs.

Fault Management

The purpose of Fault Management is to ensure the smooth operation of the networkand rapid correction of any kind of problems that are detected. Fault management

provides the network operator with information about the current status of alarm

events and maintains a history database of alarms. The alarms are stored in theNMSdatabase and this database can be searched according to criteria specified bythenetwork operator.

Configuration Management

The purpose of Configuration Management is to maintain up to date information

aboutthe operation and configuration status of network elements. Specific

configurationfunctions include the management of the radio network, software andhardwaremanagement of the network elements, time synchronization and security

operations.

Performance Management

In performance management, the NMS collects measurement data from individualnetwork elements and stores it in a database. On the basis of these data, the

networkoperator is able to compare the actual performance of the network with the

plannedperformance and detect both good and bad performance areas within the

network.

Various Interfaces in BTS

The Radio Interface( Um)Radio interface (between MS and base transceiver stations [BTS]) is the

mostimportant in any mobile radio system, in that it addresses thedemandingcharacteristics of the radio environment. The physical layer interfaces to

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the data linklayer and radio resource management sub-layer in the MS and BS and

to otherfunctional units in the MS and network subsystem (which includes the BSS and

MSC)for supporting traffic channels. The physical interface comprises a set of

physicalchannels accessible through FDMA and TDMA.

Each physical channel supports a number of logical channels used for user trafficandsignaling. The physical layer supports the functions required for the

transmission ofbit streams on the air interface. Layer 1 also provides accesscapabilities to upperlayers. The physical layer is described in the GSM

Recommendation 05 series (part ofthe ETSI documentation for GSM). At the

physical level, most signaling messagescarried on the radio path are in 23-octetblocks. The data link layer functions aremultiplexing, error detection and

correction, flow control, and segmentation to allowfor long messages on the upper

layers.The radio resource layer manages the dialog between the MS and BSSconcerning themanagement of the radio connection, including connectionestablishment, control,release, and changes (e.g., during handover).

Abis Interface (BTS to BSC)

The interconnection between the BTS and the BSC is through a standard

interface,Abis (most Abis interfaces are vendor specific). The primary functions

carried overthis interface are traffic channel transmission, terrestrial channel management, and

radio channel management. This interface supports two types of communications

links: Traffic channels at 64 kbps carrying speech or user data for a full- or half-rateradio traffic channel and Signaling channels at 16 kbps carrying information

forBSC-BTS and BSC-MSC signaling. The BSC handles the LAPD channelsignalingfor every BTS carrier. The first three layers are based on the following

OSI/ITU-T

recommendations:-

Physical layer: ITU-T Recommendation G.703 and GSM Recommendation 0-8.54Data link layer: GSM Recommendation 08.56 (LAPD)

Network layer: GSM Recommendation 08.58

There are two types of messages handled by the traffic management procedure partofthe signaling interfacetransparent and nontransparent. Transparent messages

arebetween the MS and BSC-MSC and do not require analysis by the

BTS.Nontransparent messages do require BTS analysis.The Abis interface lieswithin the base station subsystem (BSS) and represents thedividing line between

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the BSC function and the BTS. The BSC and BTS can beconnected using leased

lines, radio links or metropolitan area networks (MANs).

Basically, two channel types exist between the BSC and BTS:

Traffic channels (TCH): Can be configured in 8, 16 and 64 kbit/sformats and

transport user data

Signaling channels: Can be configured in 16, 32, 56 and 64 kbit/sformats and

are used for signaling purposes between the BTS and BSC.Each transceiver (TRX)in a BSC generally requires a signalingchannel on the Abis interface.

A - Interface (BSC to MSC)

The A interface allows interconnection between the BSS radio base subsystem and

theMSC. The physical layer of the A interface is a 2-Mbps standard ConsultativeCommittee on Telephone and Telegraph (CCITT) digital connection. The signalingtransport uses Message Transfer Part (MTP) and Signaling Connection Control

Part(SCCP) of SS7. Error-free transport is handled by a subset of the MTP, and

logicalconnection is handled by a subset of the SCCP. The application parts aredividedbetween the BSS application part (BSSAP) and BSS operation and

maintenanceapplication part (BSSOMAP). The BSSAP is further divided into

Direct TransferApplication Part (DTAP) and BSS management application part(BSSMAP). TheDTAP is used to transfer layer 3 messages between the MS and

the MSC without BSCinvolvement. The BSSMAP is responsible for all aspects of

radio resource handling at the BSS. The BSSOMAP supports all the operation andmaintenance communicationsof BSS.

The International Telecommunication Union (ITU), which manages the

internationalallocation of radio spectrum (among many other functions), allocated

the bands 890-915 MHz for the uplink (mobile station to base station) and 935-960

MHz for thedownlink (base station to mobile station) for mobile networks inEurope.

The BTS performs the radio functions of the Base Station Subsystem (BSS). TheBTS receives and sends signals through:

Air interfacefrequencies that connect the BTS to the Mobile Station (MS)Abis interfacecable or radio link that connects the BTS to the Base Station

Controller (BSC), which is the central element of the BSS

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The BTS performs the radio functions of the Base Station Subsystem (BSS). The

BTS receives and sends signals through:

Air interfacefrequencies that connect the BTS to the Mobile Station (MS)

Abis interfacecable or radio link that connects the BTS to the Base StationController (BSC), which is the central element of the BSS.

BTS mainly comprises of 2 components

Antenna system

BTS cabinet

Antenna System in BTS

REQUIREMENTS OF BTS ANTENNA

1. Strictly defined radiation patterns for accurate network planning.2. Growing concern for the level of intermodulation due to the radiation of many

HF-carriers via one antenna3. Dual polarization

4. Electrical down-tilting5. Impeccable design.

X-POL ANTENNA SYSTEM

Diversity is used to increase the uplink signal level.

There are two type of diversities:

Space Diversity Frequency Diversity

The dipoles are placed at +45/-45 to handle both horizontally and

vertically polarized components.

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Figure 4 Roof Top Antenna Setup

ADVANTAGES OF X-POL ANTENNAS

X-pol antennas reduces interference and improves indoor coverage in urban terrain. Both antenna systems can be used to transmit.

GSM BASE PANEL ANTENNA

Specification Brand: Cushcraft Model: S8802MP Frequency:880 - 960 MHz Gain:2 dBi Type:Omni

Figure 5 GSM panel Antenna Setup

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Figure 6 Radiation pattern for GSM panel Antenna

Figure 7 GSM panel antenna

Specification Brand: Cushcraft Model: S8963B Frequency:896 - 960 Mhz Gain:5 dB Type:Omni

Figure 8 Radiation pattern for GSM panel antenna

BTS Cabinet

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BTS Construction:

Below Shown in figure is a BTS with 12 TRX and supporting equipments for its

optimum performance.

RadioPartTransceiver unit is the main part of BTS and the occupancy of a BTS is termed in

number of TRX cards. Each card is responsible for transmitting and receiving thedata and is solely responsible for anything related to radio. It receives the data to

Figure 9 Base Transceiver StationBe transmitted from the baseband card, modulates it and forwards it to respective

Equipment. It receives the data caught by GSM antenna through respective

equipment, demodulates it and sends it to baseband part for further processing.

Ideally each TRX card needs an antenna for its working. By various techniques

like space diversity or cross polarization, either two or one antenna respectively isrequired for each TRX for optimum performance and coverage. A BTS can house

maximum 12TRX. So ideally we need total 12 or 24 antenna. This will increase

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the cost by 4 times plus the proximity of antennas will create tremendous

interference.

This leads to employing equipment, which will combine the signals such that less

number of antennas are used as far as possible. Such equipments are combiners,multicouplers and dual band variable gain duplex filter. Combiners combines the

transmit signal of number of TRX and forms one combine signal. There are twotypes of combiners: wideband and remote tune. Both differ in number of TRX it

can combine. Wideband can combine 2 while remote tune can combine 6.

Multicoupler duplicates the received signal and facilitates to provide the received

signal (main and diversity) to each TRX. Here also, it can handle 2 or 6 TRXsignals.Dual band variable gain duplex filter combines transmitted and received

signals intoone antenna and amplifies received signals with a variable-gain Low

Noise Amplifier(LNA). Each unit has two identical parts. Each part has one Txport and two Rx port,one each for main and diversity. The Tx port is fed by Tx outof the wide bandcombiner. As each unit has two identical parts, effectively Tx of 4

TRX are combinedinto one. Similarly in the receiving part, the Rx main and

diversity ports are connectedto the Multicoupler, which in turn duplicates thesignal and gives to TRX. Thus evenin the Rx part, four TRX can be handled.

DVGA is connected to the antenna throughfeeders. Employing of the above

equipments results into combining of Tx and Rx offour TRX into two feeders.If remote tune combiner is used then DVGA is not used. Also multicoupler which

canhandle 6 TRX has to be used.

Digital PartAfter the demodulation, the signal is sent to baseband card where the DSP occurs.

Each card can handle two TRX. Similarly in the downlink path, the data received

fromBSC is digitally processed and sent to TRX for transmission.Once the data isready after digital processing, each card gives its data to transmissioncard for

forwarding the data to BSC. The transmission card collects data and formatsit into

one E1 according to G.703 standards. It is sent to BSC via microwave link orfiber

link whichever is feasible. Similarly in the downlink path, the transmission cardreframes the E1 and distributes the data to appropriate baseband card.

Uplink and Downlink Path

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Uplink path:

From the MS, the signal is received by the GSM Antenna. The signal is given toDVGA where it is bifurcated into two combined RXs. This combined Rxs are

given toMulticouplers which further divides the signal into individual signal for

Receivedmain and diversity. This individual signals are fed to TRX whichperforms thedemodulation of the signal and gives the information signal to

Baseband card. Itperforms the DSP analysis and gives to the transmission unit. Itconstructs the E1according to the format in BOI and gives it to BSC via

transmission link.

Figure 10 The uplink and downlink path

Downlink path:

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Data from BSC is given to Baseband by transmission unit. DSP analysis is done

andthe information signal is given to TRX. The TRX modulates the signal andgives themodulated signal to wideband combiner. The combiner combines two TX

signals intoone and gives it to DVGA. DVGA gets one more combined TX signal

from otherwideband combiner. It combines both the signal and gives it to theantenna via thefeeder to be transmitted to the MS.

Skeleton Of BTS

The Base transceiver Station contains following Units:1 Transceiver unit (TSxx)

2 2-way Receiver Multicoupler unit (M2xx)

3 Transceiver Baseband unit (BB2x)4 BaseOperations and Interfaces unit (BOIx)

5 Transmission unit (VXxx)

6 Wideband Combiner unit (WCxx)7 Dual Variable Gain Duplex Filter unit(DVxx)

8 DC/DC Power Supply unit (PWSB)

9 6-way Receiver Multicoupler unit (M6xx)10 Remote Tune Combiner unit (RTxx)

11 AC/DC Power Supply unit (PWSA)

Figure 11 BTS CabinetPower Supply Unit:

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Figure 12 PWSx unit

Main blocks

The PWSx unit consists of the following functional blocks:

Power inputPower switcher

Control

Dual Variable Gain Duplex Filter :

Figure 13 DVxx Front View Figure 14 Block Diagram

The DVxx performs the following primary functions:

combines transmitted and received signals into one antenna

amplifies received signals with a variable-gain Low Noise Amplifier (LNA)The DVxx unit includes two identical duplex filter sections.Each section comprises

a duplexer, a variable-gain LNA, and an I2C-bus I/O bufferblock. Each LNA

defaults into the high-gain state at startup and can be switched to thelow-gain state

through the I2C-bus using the Site Manager.The gain of the low gain path can also

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be adjusted. The DVxx unit includesan I2CEEPROM that stores the serial number,

information about the insertion loss variationof TX filters, and other data.Thisinformation is used to compensate the frequency-dependent power variation ofthe

transmitter. The I2C-bus also carries alarm signals to indicate fault conditions

foreach LNA branch. The signals are relayed to the Base Operations andInterfaces(BOIx) unit, which generates the alarms and displays them to the user

interface.

Multi coupler

The M2xx and M6xx are passive units. The units divide Received (RX) and

Diversity-Received (DRX) signals and distribute them to the Transceiver (TSxx)

units. After receiving RX signals from the Dual Variable Gain Duplex Filter(DVxx) unit, theM2xx and M6xx distributes the RX signals to the TSxx units.

Figure 16 Block DiagramFigure 15 M2xx Unit

Wideband Combiner:

Figure 17 Front view of WCxx Figure 18 Block Diagram

The WCxx unit consists of the following components:

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one 2-way combiner

two isolators one heatsink for thermal dissipation

The WCxx unit combines transmit (TX) signals from two Transceiver (TSxx)

unitsand feeds the combined signal to the TX port of the Dual Variable GainDuplex filter unit.

Transceiver Card

Figure 19 Left Isometric View of TRX Figure 20 Block Diagram of TRX

Transceiver unit is the main part of BTS and the occupancy of a BTS is termed in

number of TRX cards. Each card is responsible for transmitting and receiving the

dataand is solely responsible for anything related to radio. It receives the data tobetransmitted from the baseband card, modulates it and forwards it to respective

equipment. It receives the data caught by GSM antenna throughrespectiveequipments, demodulates it and sends it to baseband part for further

processing.

The TSxx unit of the Nokia UltraSite EDGE BTS performs RF

modulation/demodulation and amplification for one RF carrier. The TSxx unithandles uplink

signals from the Mobile Station (MS) to the BTS and downlink signals from theBTSto the MS.

The TSxx unit consists of the following modules:Transceiver (T R X )

Frequency Hopping Synthesizer (F H S )Power Amplifier (PA )

Power Supply (PSU )

Baseband Card:

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Figure 21 BB2x Card Figure 22 Block Diagram of Base Band Card

The BB2x unit of the BTS has the following main functions:

perform s digital signal processing for speech and data channelsmanages signaling for speech functions

The BB2x also uses software downloaded from the Base Operations andInterfaces(BOIx)unit

sets its timing according to references from the BOIx unit

supports synthesized (RF ) and Baseband (BB ) frequency hopping The BB2x unithas two independent BB sections. Each section communicates with theTRX module of one TSxx unit. Therefore, one BB2x unit can process two TSxx

units,each with eight received and transmitted logical channels.

Base operations and Interface unit (BOIx)

Figure 23 BOIx Unit

The BOIx unit handles the control functions that are common to all other units in

BTS.These functions include:

BTS initialisation and self-testing

Configuration

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Operations and Maintenance (O&M) functions

Software downloads Main clock functions

Timing functions

Collection and management of external and internal alarms

Delivery of messages to the Base Station Controller (BSC) through the

Transmission (VXxx) unit

Operation

The BSC or Nokia BTS Manager downloads software to the FLASH memory of

the BOIx unit. (During download, an LED on the BOIx unit indicates the status of

the board.) The BOIx unit downloads BTS software and configuration data to otherBTS units. The BOIx unit collects alarms from other active units and saves

configuration information into non-volatile memory. The BOIx unit also controls

the uplink and down link cross-connection between the Transceiver Baseband unit(BB2x) and the Transceiver (TSxx) unit.

The BOIx unit detects unit alarms and performs recovery actions. In certain

situations, the BOIx unit resets itself. The BOIx unit generates an accuratereference clock signal for the TSxx unit, BB2x unit, and Remote Tune Combiner

(RTxx) unit. The BTS can synchronize its frame clock and number with another

BTS unit(with Talk as the clock master) or to another BTS unit. The mechanics ofthe BOIx unit provide EMI/EMC shielding for internal electrical components.

Conclusion

After the understanding of the BTS system we can conclude that BTS will be very

helpful for wireless communication. In the BSS most of the task will be done by

the BTS. Most of the operations will be done by the BTS cards and it is the base of

the wireless system. It is very helpful to the mobile communication field. It is acombined unit made up of several cards, antenna and towers (for support the

antenna).

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References

Agilent tutorial Metrohub Device Manual BTS Device Manual

Nokia BSS Systra Nokia Metro Hopper Manager Nokia BTS Manager Reflection(Alarm Manager)

Books

Mobile Communication by RAJ PANDYA Wireless Communication by Rappaport

Websites

http://www.gsmworld.com http://www.nokiasiemensnetwork.com http://www.wikipedia.org http://answers.com http://www.encyclopedia.com
http://www.wikipedia.org/http://www.wikipedia.org/http://answers.com/http://answers.com/http://www.encyclopedia.com/http://www.encyclopedia.com/http://www.encyclopedia.com/http://answers.com/http://www.wikipedia.org/

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